refactor: use f6f6f6 for surface-variant

Author: Andrew Seguin

src/material/core/m2/_theming.scss

@@ -126,7 +126,7 @@
     background: palette.$light-theme-background-palette,
     system: (
       surface: white,
-      surface-variant: #f5f5f5,
+      surface-variant: #f6f6f6,
       on-surface: rgba(black, 0.87),
       on-surface-variant: rgba(black, 0.54),
       background: map.get(palette.$grey-palette, 50),